The present invention relates to thyristors, and more particularly to an improved thyristor structure which greatly improves photosensitivity, yet still prevents the critical rate of rise of off-state voltage dv/dt from being exceeded.
P- and n-type layers are alternately formed in a four-layered semiconductor crystal structure. Generally, the four layers consist of a p-type emitter layer, an n-type base layer, a p-type base layer and an n-type emitter layer. An anode electrode is deposited on the p-type emitter layer which is the lowermost layer. A cathode electrode is deposited on the n-type emitter layer which is the uppermost layer. A gate electrode is deposited on the upper surface part of the p-type base layer adjacent to the cathode electrode. In the thyristor having the above configuration, when a trigger signal is applied to the gate electrode of the p-type base layer, a small area near the gate electrode is rendered conductive. This area expands to the entire area of the junction of the thyristor over time. When the critical rate of rise of on-state current di/dt is great during triggering, the current is concentrated only at a limited conductive area in the vicinity of the gate electrode, resulting in thermal breakdown due to a local temperature increase.
Along with the trend towards thyristors of higher dielectric strength and larger capacitance, a higher critical rate of rise of on-state current di/dt is desired without adversely affecting the thyristor's operation, while still maintaining the value of the gate control current be as small as possible. However, if a higher critical rate of rise of on-state current di/dt is used, the gate control current must generally be increased. A conventional thyristor cannot satisfy the contradictory requirements described above.
If a photothyristor which is triggered by light instead of by the gate current is used, the light energy to be used is limited and a gate drive with a higher current is difficult. Therefore, the critical rate of rise of on-state current di/dt cannot be increased.
It is strongly desired that a thyristor of high dielectric strength and large capacitance be developed which achieves an improved gate sensitivity without impairing the ability of the thyristor to operate accurately with respect to the critical rate of rise of on-state current di/dt.
Referring to FIG. 1, a prior art thyristor comprises a p-type emitter layer 10, an n-type base layer 12, a p-type base layer 14 and an n-type emitter layer 16 which is formed in an annular shape when viewed from the top outside the p-type base layer 14. An anode electrode 18 is deposited on one surface of the p-type emitter layer 10. A cathode electrode 20 is deposited on the upper surface of the n-type emitter layer 16. A plurality of pilot thyristors comprise the p-type base layer 14, the n-type base layer 12, the p-type emitter layer 10, and n-type emitter layers 22.sub.1, 22.sub.2 and 22.sub.3 concentrically disposed at equal intervals. A light-receiving portion 24 is formed at the center of the pilot thyristors.
If an optical trigger signal .phi. is radiated on the light-receiving portion of the thyristor with the above arrangement, a photocurrent Iph flows in the p-type base layer 14 transversely. This current flows into the cathode electrode 20 through a short-circuiting portion 26 disposed in the n-type emitter layer 16. A transverse potential difference established in the p-type base layer 14 due to the photocurrent Iph forward-biases the n-type emitter layers 22.sub.1, 22.sub.2 and 22.sub.3 of the pilot thyristors. When the voltage of the light-receiving portion 24 almost reaches the built-in potential of the junction formed between the p-type base layer 14 and the n-type emitter layer 22.sub.1, electron emission from the n-type emitter layer 22.sub.1 to the p-type base layer 14 is increased abruptly. As a result, the pilot thyristor near the light-receiving portion 24 is rendered conductive first. This turn-on (conduction) current flows into the second pilot thyristor through an electrode 28.sub.1. The turn-on current, as the higher gate drive current, causes the second pilot thyristor to turn on and conduct. Similarly, the turn-on current of the pilot thyristor then flows to the third pilot thyristor through an electrode 28.sub.2. As a result, the third pilot thyristor is rendered conductive. Further, the turn-on current of the third pilot thyristor flows into the main thyristor through the electrode 28.sub.3 to turn it on.
The current concentration which occurred in the initial turn-on period is dispersed by the plurality of pilot thyristors, preventing the occurence of hot spots and improving the critical rate of rise of the on-state current di/dt.
However, in the thyristor having the multi-layered amplification gate structure, the following drawbacks are presented. First, if the transverse potential difference of the p-type base layer 14 of the first pilot thyristor is increased, photosensitivity (gate sensitivity) is increased, but the gate tends to be erroneously turned on by voltage noise from the main thyristor. More particularly, if surge voltage noise is applied between the anode electrode 18 and the cathode electrode 20, a displacement current flows in the same route as the photocurrent Iph. As a result, the thyristor tend to be erroneously turned on. In other words, the critical rate of rise of off-state voltage dv/dt is the maximum rate of rise allowed which will not cause the thyristor to turn on, taking into account any voltage noise present and will hereinafter be referred to as the critical rate of rise of off-state voltage dv/dt.
More particularly, a radius R of the n-type emitter layer 22.sub.1 of the first pilot thyristor is decreased to limit the displacement current which may be generated in the n-type emitter layer 22.sub.1. Thus, the photosensitivity of the thyristor is improved without reducing the the critical rate of rise of off-state voltage dv/dt.
Secondly, if the number of pilot thyristors is increased, the later stage pilot thyristors and the main thyristor tend to be erroneously turned on due to voltage noise. This second drawback is more important than the first drawback. Since the displacement current flows over the entire junction area unlike the photocurrent Iph, the displacement current is increased toward the short-circuiting portion 26 of the n-type emitter layer 16 of the main thyristor.
Thirdly, if the number of pilot thyristors is increased, the minimum anode voltage for turning on the pilot thyristors, that is, the finger voltage is generally increased. When thyristors having large finger voltages are operated in parallel to each other, the ON voltage of the first thyristor which first turns on determines the respective anode voltages applied to the following thyristors. As a result, the remaining thyristors which have higher finger voltages than those of the conducted thyristors are not turned on.
The above drawbacks are also found in electrically triggered thyristors in addition to the photothyristors.